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What does %MAC mean?
A document said that the optimum CG position is from 13% MAC to 33% MAC. I'd like to know what that means, and what would this look like on an actual plane.

Mean Aerodynamic Chord

%MAC (Percent Mean Aerodynamic Chord) is a mathematical calculation that shows where the center of gravity is over the wing.  The calculations is rather straight forward and the generic formula is the same for all airplanes.  All one has to do is to figure out the numbers and plug them into the formula.
How is %MAC calculated?
Pilots, dispatchers or load masters will calculate the weight and balance with respect to the aircraft datum.  Unfortunately, we need to change where the reference point is from the (datum) to the front of the wing called leading edge mean aerodynamic chord (LEMAC).  There is also a trailing edge mean aerodynamic chord (TEMAC) that is calculated.  TEMAC - LEMAC = Wing width.
LEMAC and TEMAC are easy to calculate for straight wing aircraft.  It is simply the measurement from where the wing starts and ends from the datum.  For swept-wing aircraft it isn't that simple.  As a result, aerodynamic engineers figure out where the average of the LEMAC by averaging the wing root and wing tip.  The same process happens for calculating TEMAC.
%MAC is simply a calculation of how far the CG is from the LEMAC.  If the CG is at the LEMAC, we can say it is at 0% MAC.  CG at TEMAC means the CG is at 100% MAC.
In the picture below, we can eyeball the %MAC to be around 10% MAC.

Here is the generic formula:
$$ \%\mathrm{MAC} = \frac{\mathrm{CG} - \mathrm{LEMAC}}{\mathrm{WingWidth}} \times 100 $$

The formula is great because all the numbers are relatively the same for every airplane.  For example a 13% MAC means that we are most likely at or close to our forward CG limit.  This should instantly give the pilot a sense of how the airplane is going to respond versus a pilot trying to comprehend  what a CG of 2,341.0 inches aft of datum means for that particular aircraft.  The last number can be different for every aircraft type but 13% MAC will be the same.  The only difference is where the forward CG limit is located for that particular aircraft.

A little background, and perhaps an engineer can expand on the thought.  The basic weight and balance formula is $ \mathrm{Weight} (W) \times \mathrm{Arm} (A) = \mathrm{Moment} (M) $.  What may not be obvious is that we can use this formula in figuring out why %MAC is important.

Take a look at the picture above.  There is a moment between CG and CP and another moment between CP and tail down force.  These moments must equalize for the airplane to be in equilibrium.  If the CG moves forward, Moment #1 gets larger and to compensate the tail down force must increase.  We can increase the tail down force with nose up stabilizer trim.  The same analysis can be made when the CG moves aft.
If this is the case, there is a specific pitch trim setting for each %MAC and airspeed combination.  Most jet pilots will set a specific takeoff pitch trim setting for takeoff based on %MAC so that if the pilot loses an engine after $V_1$ the trim will be set for a speed around $V_2$.

Thanks! That explained it perfectly!
Could you also explain how the LEMAC and the TEMAC are determined? Because on the diagram they seem to be random lines on the wing. Thanks!

What is the center of pressure? What is the different between center of pressure (CP) and center of lift (CL)?

From my understanding, maybe someone knows more but center of pressure and center of lift can be used interchangeably.

Draw a perpendicular line from the leading edge to the trailing edge cutting the wing exactly in half.  The line will be closer to the fuselage than the wingtip as the wing is thicker at the root.  The point where that line starts, in reference to the datum, is the LEMAC and where that line ends is the TEMAC.

LEMAC is leading end of mean aerodynamic chord, similar TEMAC is trailing end of aerodynamic chord. A leading or trailing edge cannot have a chord, but a chord can have a leading or trailing end. Colloquially expanded to edge for this 2d planform concept, but "end" is correct because a chord is strictly one-dimensional.

As @abelenky points out in his comment, it is the Mean Aerodynamic Chord of the wing. This is not exactly the mathematic mean of the wings' chord, but a size which includes the damping effect of a pitching wing which scales with the square of its chord. The % part just shows that this length is given as a percentage of the mean aerodynamic chord.
Next, you need a location for this length unit. This can be the actual MAC location of the wing, other aircraft use the leading edge of the wing root as the lengthwise zero point. All measure distances in their own coordinate system, positive backwards. The airplane manual should give an exact definition how to measure the correct CG location.
If the actual CG location falls within the range given by both numbers, you are safe to go. If the CG is forward of the smaller number, your control authority might not be sufficient, the pitch control forces will be too heavy for comfort and both your rotation speed and your stall speed go up. If the CG is aft of the bigger number, your static stability is too small or the airplane might even be unstable. Also, pitch control forces will be very low so you run the risk of damaging the aircraft with too large control inputs.

A document said that the optimum CG position is from 13% MAC to 33% MAC.

A possibly better phrasing of that statement is that the "allowable CG position is..." or some such. Typically the term "optimum CG position" is used not for a range but for a given CG position that optimizes whatever you wish to optimize. Usually for transport category aircraft it's used to refer to the target zero fuel weight CG that produces the best fuel economy.
As it happens, the range 13% MAC to 33% MAC are typical landing CG limits of 747s as well as the zero fuel, taxi and takeoff limits at low weights. An often used value for the optimum zero fuel weight target for best fuel economy is 26.6% MAC. Forward of that value would mean you'd burn more fuel, aft of the value you'd burn less fuel but would be starting to have undesirable effects that outweigh the fuel economy.
If you wish to see a graphical display of such limits, go to 747.terryliittschwager.com, select any aircraft, and when the aircraft page comes up, scroll down to the OPERATING ENVELOPES at the bottom of the page. If desired you can add fuel and cargo and see how it affects the CG.


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What is the Optimum center of mass location for an air-craft?
I was doing my mathematics exploration, and i would like to know what is the optimum center of mass location of a plane.
Specifically, what would happen if the point were to be too close or too far away from the point where the lift force was acting from.
It would be helpful if you could cite a source for your information.
Thank you!

Depending on which aircraft, fuel amount and location, same for the payload. This can only be answered for a specific aircraft. By the way: CG is (for a B737) showed in percents in the FMC, not indicating a vertical location.

For limits and exceeding CG: here

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When must FAA reply to my letter?
I have sent letters requesting action and/or clarifications to the FAA medical certifications branch.  How long can FAA legally delay before they are required to answer my questions in writing?

Are you a member of the AOPA? Are you doing this in response to a medical disqualification by your AME? What class medical is this for?

@RonBeyer I don't think any of that is relevant to the question. Is a government agency legally obliged to respond to a letter? I doubt it, though the FAA Administrator might not like hearing that his staff are ignoring people.

I don't know what you're concern is about, but they are in the middle of a big rule change on 3rd class medical certifications. As far as I've heard they haven't finalized the rules yet. They were given 1 year from July 15th, 2016 to flesh out all the details

@Ben The AOPA has resources to help for members in this situation. If it is in response to a disqualification by an AME, he would have a reference number and a case file started, if he is just sending letters without a reference number, they will probably not respond. If this is for a first class medical for an airline, the airline may have resources to help. So yes, I believe the questions I asked are relevant. The FAA gets 1800+ medical inquiries per day, so unless he has some case file, he may be ignored just based on volume and cases that have references.

The FAA publication "MEDICAL FACTS FOR PILOTS" Publication #OK-06-002 states the time periods shown below. I don't know of any rule that mandates a response in any specific time period.
We will respond to your request for duplicate medical certificates within 3 working days of receipt.
We will respond to your request for full review of medical records within 30 working days of receipt of all the necessary records and testing.

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Why are smaller wings in the front with these VTOL aircraft?
I would think it would be better to put the larger wings under the front or middle closer the the payload/center and have the smaller wings in the tail? Why would these designs do the opposite?

With the first (DAPRA's X-plane) you can see the weight is going to be put properly under the bigger wings but I also wonder: why have it fly with the smaller wings infront?
If a teardrop shape is the most aerodynamic then the big wings should be in the front. Also it looks like a having the smaller wings in the back would increase stability in flight.

The second picture is of a concept plane that is severely flawed in its design.

These are both concepts so its hard to say but they look like canard's like those found on the work of Burt Rutan

That's Daniel Wiegand's Lilium. I don't think the guy is dumb though.

@DanielCaoili: See https://aviation.stackexchange.com/q/35436/520 for some discussion of the Lilium.

I guess he is dumb lol

It's because with the canard configuration, the smaller stabilizing surface is a lifting surface instead of the down force surface of a conventional tail.
(A regular tail airplane is a seesaw with someone sitting on one end and another pushing down at the other; a canard airplane is more like a table with most of the weight at one end where the fat legs are and skinnier more lightly loaded legs holding the other end up.  Canards get their pitch stability from having a steeper lift slope at the canard surface, whose airfoil is chosen to achieve that purpose, so the nose will rise with increasing speed and drop with decreasing speed).
So if you want to as much vertical thrust as possible from the available flying surfaces, it makes sense to use the Canard configuration where all the surfaces are lifting.  Canards are nothing new; the Wright Flyer was the first.  You don't see them everywhere because there are significant limitations, but in a VTOL configuration like this, the Canard layout is mostly a plus.

You don't see canards everywhere because they are actually less efficient, for aerodynamically stable configuration.

Yeah when the Varieze came out in the 70s it was kind of touted as a super-plane by homebuilt standards but they really aren't much faster than regular tailed airplanes with similar construction.  Then there was the Starship fiasco, which confirmed for everybody that the concept was an aerodynamic dead end.

If the canard configuration gives maximum lift (vertical thrust) why is it not more commonly used for high endurance aircraft, for instance: Solar Impulse

It's not so much that but the fact that if you are going to use lifting thrusters that are integrated into the wings, you might as well have use a config that is all lifting wings.  You couldn't put them on a downward lifting tail surface.  In any case the theoretical efficiency benefit of the canard is lost in the noise of a bunch of other factors and there are lots of disadvantages, which is why nobody uses it today in mainstream a/c.

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Aviation business administration/ management or aeronautical engineering
Hello im an  a & p considering continuing education in avaiation field for better opportunity, wondering if there are any suggestions on either Aviation business administration/ management or aeronautical  engineering wold be better based on salary and opportunity wise?
thank you!

I'm afraid this is either off-topic or too broad. Aviation business admin/management can be a number of different, entirely unrelated fields, and aeronautical engineering covers a wide range of job types. I'd suggest doing some research on job boards in your area to get a feel for the demand and salary ranges.

Also there doesn't appear to be a clear question.

Both areas have good opportunities for work and good pay, the engineering side of things offering a little higher salary but still....
As in any occupation, I would seek something that you feel fulfilled in doing more than merely making a good living.  I left aerospace engineering and went into software development for that reason.  You have to spend 1/3 of your adult life working so make sure it's something you can go home at night with a sense of accomplishment and fulfillment in doing.  Don't work a job where you're miserable and living for the weekends simply because it pays a little better.
Ask yourself if you enjoy the managerial aspects of aviation or the technical aspects.  If you enjoy working on aircraft try getting a job as an A&P at an OEM and find ways to learn about becoming a manufacturing engineer.  I know several people at Boeing and Gulfstream who followed that route and it would be a good way to get into the engineering side of the business.

I think this is a valid question as it has to do with a career in aviation.
There is a right answer to this question and it's aero engineering.  You can take it anywhere (and be taken seriously) and coupled with an A&P could be attractive to major aerospace employers.  It also carries weight in non-technical roles whereas the biz degree will generally keep you in non-tech positions.
If you're going to do business get a plain ol' BS in Business Admin.  Some specialty like marketing or finance would be better but do what whatever you can for as little money as you can.  Aviation is a specialty that nobody cares about, even within the aviation community!  If you have career experience already then the degree is only going to check a box on the application.  Consider a Tier I MBA if you grow your career into a management position.  Undergrad degrees on their own, even from great schools, are sort of like highschool v2.0.
You have the advantage of work experience which makes the "name recognition" of your undergrad school less important.  I would get the most reasonably priced degree you can and focus on a science/engineering path.  The name of the school becomes more important when you consider a masters/doctoral program.
Context for the answer:
I have a BS in BizAdmin and a Tier I MBA.  I started my undergrad as a "professional flight" major and switched to the university's business school within 18mo.  I considered a transfer to the avation management major but the course list was nearly identical to the "pro flight" coursework.  Additionally, the aviation management program was under a college nobody would recognize whereas the business college is well ranked.  Price was equal.  If I could do it again I would consider a mechanical engineering path because it's more fun than business.
Food for thought.


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Are ducted fans more efficient?
As far as static thrust, does a shrouded/ducted propeller outperform an un-shrouded propeller? It seems that the guide blades on the bottom of the duct would allow little more thrust to be extracted from the vortex the prop generates by redirecting the air flow. Also the duct seems like it could "focus" the propelled air (like a rocket thruster) resulting in better thrust.
(Source)
Of course I can see how an improperly implemented duct could really kill efficiency (creating more interference and turbulence), but what about a well designed one? And if they are better, why wouldn't we seem most propeller aircraft use them (what is the advantage about not having the duct)?

Related, but not for model aircraft: What are the advantages and disadvantages of ducted fans in designs such as the Airbus E-Fan? --- and --- Would connecting propeller blades with a continuous ring reduce induced drag? --- and --- Rings around propellers

Imagine running on a carpet of ball bearings. That is what happens when the air at the tips of the propeller goes in circles through the blades repeatedly. The shroud stops that.

there is a lot going on here.  Short answer is that a ducted fan (what you have pictured) can produce a lot more thrust (experiments from one paper say twice as much) than an open rotor of the same size.  The big thing that these ducted fans have going for them is the fact that tip losses become negligible because there simply isn't room for the tip vortex to develop because there is a duct in the way!  Simple, easy solution that works well.
However, nothing comes for free.  One, you have to have the duct there, which is weight & money (you have to build the duct and the support structure, which adds some drag too).  Think of scale too--as propellers get larger, the duct has to grow with them.  Imagine the difficulties in putting a 20' duct on a B-36 propeller!
Second, the manufacturing tolerances on the duct are critical--the tighter it is, the more efficient you are (to a limit, naturally)...but the tighter it is, the more susceptible you are to having your blades strike the duct due to flexing of the duct or the blades.  And, speaking of flexing blades and the necessary support structure you'd need, this could make pitch control (on a propeller, variable pitch, and on a rotor, cyclic and collective) difficult to implement, depending on which of those schemes you're looking to do.  Then again, for a small implementation using RPM control...maybe you don't need those anyhow.
Similarly, while a crosswind on an open rotor leads to a sudden decrease in power, a crosswind on a ducted fan leads to a sudden pitching moment due to the lip of the duct and the asymmetric inflow to the fan (which, since you're now producing more thrust to begin with, asymmetric inflow effects become even more pronounced).  Now, if you stick this on the front of a typical Cessna 172, say, and throw a crosswind at it (which, if you think of takeoffs and landings, which since they must be in a certain direction, having a crosswind is actually quite likely) imagine the yawing moments that would be induced by the propeller (proportional to the static thrust) -- even before the aircraft got off the ground and before you have a lot of control authority to counteract it!  A similar scenario in the air could be as dramatic.  And, yeah, you could throw some guide vanes at it to help, but nothing comes for free.  Guide vanes, which is a workable solution, now mean that you need to have actuator(s) for those vanes, power & maybe hydraulics for the actuator(s), mounts for the actuators, hinges for the vanes, linkages from the actuator(s) to the vanes, redundancy for the actuation or some means of facilitating control in an emergency...it's a lot of work.  Or, by contrast, you could just bolt a fixed pitch prop on a shaft at the front of said Cessna 172 and call it a day.
In the long run, and this is some speculation, I think it just leads to a more complex system, particularly in full-size implementations, which is a maintenance, cost, and manufacturing headache that isn't worth having if you can get plenty of performance from a regular open propeller.  If you venture into the world of diffuser augmented wind turbines, there have been similar debates going on for decades...and a ducted wind turbine has yet to be successful on the large scale.  Similarly, while ducted fan aircraft have flown...

(Ryan XV-5A Vertifan, circa 1950.  The ducted fans are underneath the circular domes on the wings and another under the shuttered area on the nose for pitch control. Source: http://www.aero-web.org/specs/ryan/xv-5a.htm)
...there aren't any non-MAV examples around today that I know of.
Source:
https://theses.lib.vt.edu/theses/available/etd-05262005-170916/unrestricted/WGraf_Thesis_2005.pdf

Please source your image, as well. It's not clear if it comes from the thesis paper, or somewhere else.

Got it--thanks!

The Edgley Optica is a good example of a flying civilian ducted-fan aircraft.

So what do you think about adding only guide vanes without the duct?

I'm not sure ducted fans were used in the XV-5A Vertifan. These are lift fans driven by gas from the engine (which is not on the wings) and using the circumference of the fan as a turbine. The duct is actually the turbine annular duct, I don't know if it is also used as a fan duct.

Looking at this image (http://prototypes.free.fr/vtol/lift-fan/xv5_22.jpg) and listening this video made by Ryan (https://www.youtube.com/watch?v=fg9guWqKShs -- go to 1:21) it seems like the engines in the fuselage produce high-speed air that is diverted to the edge of the fan duct.  This air then drives the fans at sufficiently high speed to create a lifting force from the motion of the fan rotor.  Looking at the wing structure in these videos and at the exhaust routing, I don't see how else to get the vertical lift another way either.

Yes, that's what I was saying. But are the fans ducted? I'm not sure as the "duct" we see seems to be the duct where engine air circulates to turn the fan.

Ah, I see.  My apologies.  Excellent question...it's hard to tell.  The image I linked to isn't terribly descriptive, but this might be better: go to 1:36 of https://www.youtube.com/watch?v=jUnUe9dBKAI&t=3s.  It appears that there is some deliberate shaping of the casing around the lift fan, which would indicate to me some intention to increase mass flow rate through the fan.  I can't tell the duct geometry around the fan itself for sure because of the shadows, but I could believe that at least part of the fan is shrouded.  What qualifies a duct for you?

A ducted fan can be designed to produce more thrust at the same fan (and same diameter) than a propeller -- or it can be designed to produce the same thrust at lower power.
However, it is not likely to be achieved with a very short duct like the one you have pictured.  That is more of a shroud that merely protects people from the blade tips.
The longer you can make your duct, the more you will be able to take advantage of this aerodynamic phenomena.  It also helps to have a nice rounded inlet to the duct -- often called a bellmouth.  I would look for a duct length-to-diameter ratio of at least 0.5 for a reasonable augmentation.

Another thing to keep in mind is that ducted fans have much more than 2 blades. Increasing number of blades decreases the propeller/fan efficiency. The main advantage of increasing number of blades is to get higher thrust density, i.e., getting more thrust with the same propeller/fan diameter. So, what ducted fans really do is: the multi-bladed fan provide a lot of thrust density and the duct try to compensate the efficiency lost. Hopefully, the ducted fan can achieve the same efficiency of the 2-bladed propeller...

Firstly, welcome to Aviation Stack Exchange @HenriqueMoritz. As for your answer, you definitely have a valid point with your explanation that increasing blade count generally improves many factors of efficiency, but nonducted and ducted fans can have similar to equal blade counts. The question in specifics is asking about the aerodynamic effects of the duct itself, which iirc offhand, mainly contributes in the area of thrust loss at the tips, which leads to issues that are countered by the ducts. Thrust density does improve with ducted propulsion though, so I see what you're saying here.


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How to calculate airfoil geometry with the maximum lift to drag ratio for a given Reynolds number?
How does one determine the optimal cross sectional geometry to maximize Cl/Cd?

Basically I want to be able to know (or at least get a general idea for) the best airfoil geometry for a given airspeed to get the right lift with the smallest drag

Are there any equations that can relate these?

The optimum airfoil is a cambered line of zero thickness. If operated at just the correct angle of attack, it will produce the highest L/D possible. It's cross section is trivial: It has none. However, it will not allow to build a clean wing since it has no space for a spar. It would need to be kept in shape by lots of wire bracing, and then the overall L/D of the resulting aircraft would be horrible.
Next, if the angle of attack is only slightly different from the optimum value, the sharp leading edge will cause early separation on one side, causing the L/D figure to plummet. This airfoil will not be practical for any purpose other than establishing a theoretical optimum figure.
Real airfoils need a blunt nose and some thickness. There are plenty enough airfoils to choose from already. All of them were designed not just for one single design point, but as an overall compromise to cover a wider range of parameters in real-world operation.

I suppose this optimum airfoil should have the most camber possible that maintains laminar flow over the surface.

@DanielCaoili: Depends on the Reynolds number. Most likely it will be laminar for 30% - 70% on the upper and 100% on the lower side. The turbulent transition is needed to achieve a steeper pressure recovery - with laminar flow alone you simply cannot get much lift out of the airfoil.


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What is the difference between AIM, AIP and AIS in the US?
I know that in the US there exist documents which are published by the FAA such as the Aeronautical Information Manual (AIM) and the Aeronautical Information Publication (AIP). What is the difference of each one? What is their relationship with the Aeronautical Information Service (AIS)?

An AIP, e.g. Brazil AIP, is the local implementation of ICAO annex 15 rules and encompasses aeronautical information like airport information, approach plates and routes, AIS is the information service provided by ICAO members, like NOTAMS and circulars. AIS includes AIP publication. AIS is currently transiting to AIM to use more modern means of communication.

From the beginning section of the Aeronautical Information Manual (AIM).

This manual is designed to provide the aviation community with basic flight information and ATC procedures for use in the National Airspace System (NAS) of the United States. An international version called the Aeronautical Information Publication contains parallel information, as well as specific information on the international airports for use by the international community.
This manual contains the fundamentals required in order to fly in the United States NAS. It also contains items of interest to pilots concerning health and medical facts, factors affecting flight safety, a pilot/controller glossary of terms used in the ATC System, and information on safety, accident, and hazard reporting.

Aeronautical Information Publication front section

2.1 The AIP is prepared in accordance with the Standards and Recommended Practices (SARP) of Annex 15 to the Convention on International Civil Aviation and the Aeronautical Information Services Manual (ICAO Doc 8126). Charts contained in the AIP are produced in accordance with Annex 4 to the Convention on International Civil Aviation and the Aeronautical Chart Manual (ICAO Doc 8697). Differences from ICAO Standards, Recommended Practices and Procedures are given in subsection GEN 1.7.

Aeronautical Information Services

Is the authoritative government source for collecting, validating,
storing, maintaining, and disseminating aeronautical data for the U.
S. and its territories.
Develops and maintains all public instrument flight procedures and airways.
Serves as the FAA's aeronautical charting authority for the development, publication, and dissemination of aeronautical charts and products to support aviation and to meet demand for increased capacity, efficiency, and predictability in the airspace, routes, and airports of the NAS.

authors: Тимофей Свирин


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What is the meaning and difference between CTAF, UNICOM and MULTICOM in the US?
What is the meaning and difference between CTAF, UNICOM and MULTICOM in the US? Could you please give examples of radio communications with the correct phraseology to be adopted in each case?

From the Pilot/Controller Glossary

UNICOM− A nongovernment communication facility which may provide airport information at certain airports. Locations and frequencies of UNICOMs are shown on aeronautical charts and publications.

For example, an FBO may have a unicom frequency that they use to provide information to incoming aircraft and provide traffic advisories (not to be confused with instructions), wind directions, weather, and preferred runways. Unicom frequencies have a ground station at the airport in question.
It is important to note that when talking on a Unicom, the person on the ground that you are talking to in the FBO or airport office is not an air traffic controller.
An example of talking on this would go something like:
Pilot: "KXYZ Unicom, N12345 request traffic advisory"
Unicom: "N12345 No traffic in the pattern, winds XX @ YYY, recommend runway ZZ"
Pilot: "KXYZ Traffic, N12345 will be entering the 45 downwind for runway ZZ"
And so on just like a CTAF if the unicom/CTAF are the same, otherwise switch to the CTAF and make announcements.

MULTICOM− A mobile service not open to public correspondence used to provide communications essential to conduct the activities being performed by
  or directed from private aircraft.

This is an advisory frequency that does not typically have a ground station associated with it. Pilots are to self-coordinate on the Multicom frequency but there is no ground station at the airport for that frequency. Often many small uncontrolled airports will use the same multicom frequency so you can hear traffic at multiple airports while on a mulitcom.
This is the typical position/intention reports that you normally make at untowered fields, only you are typically just broadcasting your intentions. Sometimes you may have to work with other incoming aircraft such as reporting each other in sight or telling somebody you'll extend downwind to follow them in, etc.

COMMON TRAFFIC ADVISORY FREQUENCY (CTAF)− A frequency designed for the purpose of carrying out airport advisory practices while operating to or from an airport without an operating control tower. The CTAF may be a UNICOM, Multicom, FSS, or tower frequency and is identified in appropriate aeronautical publications.

Usually CTAF is used as a multicom, but there are cases where it shares the unicom frequency. If the field is towered and in operation, you'll be working with the tower to coordinate your actions. If the field is non-towered or the tower is not in operation, you'll make position reports and work in the frequency just like a multicom.


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What is the difference between the FAA FNS NOTAM and the FAA PilotWeb NOTAM websites?
I figured out there are currently 2 different FAA websites for NOTAM online search:

FAA FNS NOTAM website available at https://notams.aim.faa.gov/
FAA PilotWeb NOTAM website available at https://pilotweb.nas.faa.gov/

What is the difference of each one? Which one is better to use? Do both of them provide information about Distant (D) NOTAMs, Local (L) NOTAMs and FDC NOTAMs?

I got this response via email from the FAA:

PilotWeb is considered a Legacy Application based on the USNS, United States NOTAM System. The FNS, Federal NOTAM System, NOTAM Search is a new application. Many Legacy applications will eventually be replaced with the newer FNS applications over the next year. Both applications should be able to provide you with the information you are looking for.

Can you give a link to that quote?

It was a FAA response that I got via email.

Thanks, I was wondering because the last page updates at the FNS link are from 2014 (the NOTAMs themselves are current, though) so it was hard to say if the site is actually in use or not. I suspect the key word from the email is "eventually" :-)


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Who is responsible for airspace separation when an IFR flight is cleared for a visual approach?
In areas tightly packed with airports (FXE, PHF, and countless others) it is possible to enter the airspace other than that of your destination airport. Some are less than 5 miles apart.
In particular,  say you are on an IFR flight plan and cleared for a "visual approach" to your destination.  While proceeding with the visual approach and descending, you could enter the class D airspace of a nearby airport.
You are in communication with the destination tower but not the tower of the other airspace you have just entered.
Is it the pilots responsibility to avoid the surrounding airspace or the tower controllers responsibility to coordinate with the other towner when IFR and conducting a Visual approach as referenced in AIM 5-4-23.

Related: What are the pilot responsibilities for "maintain visual separation" clearances?

Under IFR the pilot is responsible for flying according to her assigned clearance and for obeying controller instructions (including frequency changes).  The controllers are responsible for coordinating any necessary airspace transitions. The airspace classifications one sees depicted on a VFR chart are mostly irrelevant to IFR flight, and even to ATC - the controllers have separate agreements about who controls each sector of airspace, and those don't necessarily line up with the class B/C/D divisions.

Can you provide a reference for this opinion?

@Lnafziger, 14 CFR 91.123 is pretty straightforward IMO: once an IFR clearance has been obtained, a pilot must operate according to that clearance. It's no longer their responsibility to follow the Class B/C/D entry rules because they've been cleared to fly their assigned route no matter what type of airspace that route enters. This extends to a clearance to fly a visual approach. ATC will either issue restrictions to the pilot to avoid airspace they don't own, or coordinate with the controller who does own that airspace.


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How does a dispatcher assign relative costs to trajectories in TOS?
In FAA's Combined Trajectory Options Program (CTOP) flight operators submit a set of possible routes (TOS—Trajectory Options Set) that the airline is willing to take for a given flight, if there is a flow constraint present in the air space. The airline tells the command center, which trajectectories it prefers by assigning a relative trajectory cost (RTC) to each route, which tells the command center how much ground delay the airline is willing to take to get a certain route.
My questions are - how do dispatchers determine how much RTC to assign to each route? What are the main things that they consider? Is there some kind of software that allows to calculate that, or is the process relatively informal and intuitive?


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In what ways do air traffic controllers communicate with pilots in class A airspace?
I know they use two-way radio, but is that the only communication pilots receive from air traffic controllers? Also, in general in class A airspace, how often do pilots hear from air traffic controllers?

Hello Miles, welcome to Aviation.SE!

A VHF or UHF radio.
Class A airspace operates under IFR, so pilots must be on a filed light plan, cleared by ATC and maintain two way communications with ATC at all times.
That and hand signals don't work too well at 33,000 feet.

VHF & UHF in domestic airspace, HF transoceanic. Digital communications Controller/Pilot Data Link (CPDLC) have been used over water for some time and are in the process of being implemented domestically as "Datacomm". Over water is satellite communications while domestic digital will be carried over ARINC/SITA cell phone type networks.  Datacomm is being fielded for Clearance Delivery at large terminals first with en route communications to follow.
As far as the towers go, I think they may still have light guns for signaling aircraft that lack radios.


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To what extent is the HOST system still in use?
The Host Computer System (HOST) used to be the backbone of the US air traffic control centers (ARTCCs). There was a lot of publicity early in 2015 about the En-Route Automation Modernization (ERAM) fully replacing it, as the FAA announced they were finally finished implementing ERAM. However, deep in the FAA website, the HOST system still seems fairly active.
I just want to clarify if it is fully out now. Is it still available as a backup? Do certain ATC systems still draw from HOST systems? Do controllers still use certain HOST systems?
Overall this is a pretty technical question, but any information is helpful.

There are descriptions of the various air traffic control systems in the faa website, but it takes a login to view them. Basically, they imply (and also explicitly say) that the HOST system is still in service. I think they must still be decommissioning it.

I was lucky enough to work a career that spanned all three systems:IBM 9020, IBM 3083 (HOST) and ERAM.
HOST is no longer in use even as a backup. The backup system used to be a stand alone system called DARC (Direct Access Radar Channel). ERAM has a redundant channel of supposedly identical hardware and software as a "back up". This is problematic though. I believe the current ARTCC system uses the original DARC software hosted on the display channel (DSR) as backup.

Between the IBM 3083 and ERAM, there was the original DSR system, which used the HOCSR (HOST/Oceanic Computer System Replacement) using the token-ring setup with an IBM RISC 6000 at each control position. The 6000s were originally part of the Sector Suite fiasco, but were released back to IBM. They sat in a warehouse for 12 years, then IBM sold them to the FAA again for DSR.

Actually, DSR was Display System Replacement which was the replacement for Central Display Complex (CDC). CDC did nothing but do graphics processing. The original 9060, HOST (IBM 3083) and ERAM did all the flight data and surveillance processing.

@lain, you're absolutely right, but I'll leave my comment up, for the RISC 6000 (they really did get sold to the FAA twice). Were you automation, perhaps?

I was ATC but got into the automation stuff mid career just out of curiosity. I did a few years contracting as an ERAM subject matter expert when I retired from ZSE.


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Does ground effect scale 1:1 with wingpsan?
The rule I have been taught is that ground effect begins within 1 wingspan of the ground.
What is the derivation of this measurement?  Is it actually true, or just an urban-legend/rule-of-thumb style teaching aid?
If it is a valid way to assess where ground effect begins for any given airplane, what limitations does it have?  Do particularly heavy or airplanes with exceptionally long wingspans not fit this rule the same as a smaller more common build of aircraft?

It's most definitely not urban legend. However, it also does not scale 1-1 with wingspan. Rather, it scales nonlinearly with above ground height to span ratio ($h/b$). The simplest analytical prediction for ground effect can be derived from potential theory and the circulation theory of lift.
We know that the wing can be modeled as lifting line consisting of a line of vortex bound to the wing and trailing vortices that look like horseshoes. The trailing vortices induce downwash on the wing and everywhere else. But what about at the ground directly underneath the wing? Due to the tangency boundary condition of non-viscous flow, induced downwash must necessarily be zero everywhere on the ground.
Within the context of circulation theory of lift, the only way we can achieve this is if we have a mirror wing underneath the ground with the ground plane being the plane of symmetry, like this:

Since the image wing is exactly inverted, it produces upwash that reduces the overall downwash on the real wing. In the super simplified case of a single horseshoe vortex (which is incorrect), after some maths, the induced downwash at the wing root is now:
$$w_i=-\frac{\Gamma}{\pi b}\frac{16(\frac{h}{b})^2}{1+16(\frac{h}{b})^2}=w_{i_\infty}\frac{16(\frac{h}{b})^2}{1+16(\frac{h}{b})^2}$$
where $w_{i_\infty}$ is the downwash induced without the ground image (i.e. free-air). Turns out the latter factor is an ok approximation of total reduction of induced drag in ground effect as well. As you can see, at half wing span away from the ground, we have 80% of the free-air induced drag; at one full wing span away, we are at 94%. The effect falls away quickly.
You can read this MIT lecture on ground effect (from which I've taken the above diagram) for the derivation of the simplified ground effect prediction. More precise ground effect accounting can be found in references such as Raymer and ESDU Item 72023.

Even though the derivation is ok, the conclusion is misleading. Please see my answer.

@Zeus you do know the h/b relation hold in Raymer, Asselin, Roskam, wind tunnel tests and flight tests right? For conventional configurations, I don't see why you would say it is misleading.

For a given configuration, it holds for h/c as well, just with a different coefficient. It is misleading exactly the way the OP got confused: if the wingspan is the factor (as is often explained to pilots), then longer wingspans will experience ground effect at a higher altitude, right? Wrong. On the contrary, for an infinite wingspan there will be no meaningful ground effect at all. If you try to land two similar airplanes with substantially different aspect ratios (say, PA-28 vs DA-40 or SR-20), you'll notice that shorter wing experiences more change.

...So, instead of saying "ground effect begins within 1 wingspan of the ground", it is more meaningful to say something like "ground effect begins within 10 chords from the ground".

@Zeus If you have a large span, then ground effect will start showing up at a higher altitude. If you have infinite span, it will start at infinity. It'll also have zero induced drag to begin with. Why is this contradictory? I haven't seen ground effect being linked to h/c for standard wing configurations. Do you have any reference?

Well, the whole body of the Russian aerospace literature could be my reference, but it is obviously in Russian. The nomenclature is also different. I cold find you a few credible examples online, but I'd need to translate. Perhaps try this for start, but automatic translation is very crude.

@Zeus Your source doesn't make sense to me. It says GE is proportional to chord of the wing, but then says it's proportional to diameter of the rotor.

This is an encyclopaedia article which tries to cover both the fixed and rotary wing.

With respect to the contraditcion, just think of a very large but finite wingspan (a glider) vs a short wing of the same area. The glider wing will indeed have less things (induced drag) to play with in the first place, but the point is that you will start to have noticeable change (say 5%) (which is the definition of the threshold) at a lower altitude than with the short wider-chord wing. This is a plain fact confirmed experimentally. Of the famous examples, of all the airliners Concorde had by far the greatest GE - so much so that it didn't need a flare for landing.

This is nothing more than a rule of thumb, and a rather poor one.
As Jimmy's answer correctly states, the ground effect scales expotentially with the height-to-span ratio, and the effect is primarily on reduction of the induced drag.
However, this approximation apples only to a given fixed aircraft. It often leads to incorrect conclusion that the wingspan itself is involved here, and the original 'one wingspan' rule of thumb reinforces it. The uneasiness expressed in the last paragraph of your question shows that it is easy to feel that it must be wrong. Unfortunately, this is a very common misconception.
The truth is, ground effect is negatively related to the wingspan, and positively related to the wing chord. The wing chord, or more accurately, the height-to-chord ratio $h/c$, is a better factor to use for approximations.
Of course, for a given aircraft (wing) with a fixed ratio of $b/c$ (read aspect ratio), one can express the effect based on either wingspan or chord. But wingspan is misleading.
Indeed, if we double the wingspan, and accordingly halve the chord, keeping everything else (particularly lift and height) the same, what will happen to the ground effect? It will reduce, contrary to the 'wingspan' rule.
This does not directly contradict the derivation in the Jimmy's answer; rather, one should remember that the induced drag ($w_{i_\infty}$) will also reduce in this case. As we approach infinite aspect ratio, both the induced drag and the ground effect will tend to zero. The near-zero chord clearly and intuitively indicates that.
You may notice that aircraft which rely on ground effect always have stubby wide-chord wings. The reason is exactly that: the height at which the ground effect becomes noticeable depends on the chord rather than the wingspan, for a given lift. Interestingly, in the Russian aerospace school (and USSR/Russia is known for its ekranoplans), wingspan has never been used as a proxy for ground effect calculations. Only the wing chord.

"If we double the wing span and halve the chord, we reduce the ground effect." How did you reach this conclusion? Ground effect is expressed as a factor on the free-air downwash; a reduction in free-air quantities themselves due to aspect ratio change doesn't factor into this.

This is a very narrow definition of ground effect, but even then, with the same mirror wing model, it is obvious that this ground effect will not increase with the wingspan (for the same lift and other conditions). Generally speaking though, ground effect is the overall change of all aerodynamic characteristics due to ground proximity. And in this sense (most importantly for pilots), reduction of total drag (and increase of L/D) near ground will inversely depend on wingspan (and positively on chord).


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What is the FAA's official outlet for informing the public of permanent, systemic changes?
When the FAA changes a regulation or rule, such as the recent change in April that student pilot certificates will be a plastic pilot certificate without an expiration date... (This page)

Student pilot and medical certificate are no longer the same document, therefore, refer to 14 CFR 61.23 for complete information on duration of a medical certificate.
After April 1, 2016, Student pilot certificates do not expire; the certificate will be surrendered and superseded upon successful completion of the higher certification. Student pilot certificates issued prior to April 1, 2016, will expire according to their expiration date, either 24 or 60 months from the date of issuance.

... what is the standard, official, go-to document to keep abreast of the existence of these new changes?  How often is is released or how are we notified of its release if it is irregular?
This question is not about NOTAMS which cover temporary changes, or changes that will later be added to a document with regularly occurring updates at known intervals such as those for approach charts.
This question is also not asking about the contents of advisory circulars; however, I would view an update to an advisory circular or release of a new one as being a pertinent, permanent change considering many of them have been around since the 1970s and contain official FAA information on various topics.
So, to be totally clear, I am asking how we are notified when there are changes and where they are located not necessarily what the specific, full-text contents of the changes are.
The changes could be to such things as:

Handbooks and manuals
Federal Aviation Regulations
Advisory Circulars
Practical Test Standards
The Airman Information Manual
FAA knowledge exam contents

The FAA has multiple email groups you can subscribe to.
https://public.govdelivery.com/accounts/USAFAA/subscriber/new?topic_id=USAFAA
Click the link and you will be brought to a page where you can provide your email address.

You will then be brought to a page where you can select the email groups you want to subscribe to.

Great answer, but I am very sad to see it.  It would seem like this method might potentially "over inform" a person and cause things to get lost in the weeds. Regardless, I'm going to give it a try and hope I don't wake up to 50 e-mails from the FAA tomorrow morning.

I get between 4 and 6 emails a day between what I subscribed to.  Most I don't care about but always read the bold text and decide.

authors: Χρήστος Αυγένης


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What is an air traffic controller's role in handoffs?
I know they tell the pilot to tune into the new controller's frequency, but do they also have to do technical stuff? I'm trying to get a sense of how automated hand-offs are. For example, do controllers have to send flight progress strips to the next controller or is that done automatically?

If you are inclined to, you may read the AF447 accident report, it explains how the transfer between Atlantico and Dakar controllers should have happened and what are the responsibilities of a control center in the handoff, including in the rescue process.

In most modern ATC units, almost all transfers are conducted as “silent transfers”. A silent transfer is a transfer where all necessary information about a flight is sent in advance to the next ATC sector by an automated system. In this case, the controller does not have to manually talk to the next sector, but will simply instruct the pilot to switch to the next frequency prior to the area of responsibility border.
Where manual coordination is required, it is most likely done by the planning controller (PLC). As opposed to the executive controller (EC), who talks to the planes, the PLC (who will be seated right next to the EC) handles coordination with adjacent sectors via dedicated telephone lines.

There are two main parts of a transfer: the transfer of communication and the transfer of control. The transfer of communication is the actual frequency switch – the instruction for the pilot to switch from one frequency to another. The transfer of control is the transfer of responsibility of the flight from one controller to the next.
Before answering your question directly, let us establish the premises of the question.
Note: this answer refers primarily to European procedures, and is largely based on this EUROCONTROL document
In respect to your question, the transfer of control is the interesting part.
Firstly, we must realise that there are two fundamentally different premises for a transfer of control. Either the transfer takes place from one position/sector to another position/sector within the same ATC unit (for example between two area control sectors in the same area control centre), or the transfer takes place from one ATC unit to another (for example when a flight crosses a state border).
In the first case, the adjacent sector will typically already have access to information about the flight, since both sectors rely directly on the same data source. In the second case, this might not be the case.
Additionally, in the first case, controllers will often be physically seated right next to each other. For units where paper flight progress strips are still used (mainly some approach and tower units), this enables controllers to physically pass the flight progress strip to the next controller.
In both cases, there are four required stages which lead up to a transfer of control (refer to ICAO document 4444, section 10.1):

a) Notification of the flight in order to prepare for coordination, as necessary;
b) Coordination of conditions of transfer of control by the transferring ATC unit;
c) Coordination, if necessary, and acceptance of conditions of transfer of control by the accepting ATC unit; and
d) The transfer of control to the accepting ATC unit or control sector.

Doc 4444 also states that:

ATC units shall forward from unit to unit, as the flight progresses, necessary flight plan and control information.

Now, your question is, to what extend is this data transfer process automated. ICAO document 7030 gives us a clue to the background of the answer:

When so agreed between adjacent ATC units, a computer-assisted coordination process shall be introduced to eliminate the need for verbal coordination of boundary estimates and to reduce the amount of manual data input into ATC computers.

In most modern ATC units, almost all transfers are conducted as “silent transfers”. A silent transfer is a transfer where all necessary information about a flight is sent in advance to the next ATC sector by an automated system. In this case, the controller does not have to manually talk to the next sector, but will simply instruct the pilot to switch to the next frequency prior to the area of responsibility border.

Silent transfer of control procedures require that traffic is predictable and sufficient system support is provided to ATCOs to ensure:
-They are in possession of all elements related to the aircraft profile and
-Should the agreed conditions of the transfer change, they have the means to speak to each other such that new conditions are agreed expeditiously and safely.

Silent transfers are based on the exchange of flight data, which happens through connected Flight Dara Processing Systems, which make use of the OLDI protocol.

The operational use of connections between Flight Data Processing Systems (FDPSs) at ACCs for the purpose of replacing verbal "estimates" is referred to as On-Line Data Interchange (OLDI). The implementation of basic OLDI messages is mandatory in accordance with Regulation (EC) No 1032/2006. These messages are defined by the EUROCONTROL Specifications

What this regulation means in practise is that, within Europe, silent transfers are used more often than not. However, you will also see units where a manual coordination prior to a transfer is required, for example flights crossing the border from certain European countries to certain non-European countries. Where manual coordination is required, it is most likely done by the planning controller (PLC). As opposed to the executive controller (EC), who talks to the planes, the PLC (who will be seated right next to the EC) handles coordination with adjacent sectors via dedicated telephone lines.

In most countries the coordination prior to hand-off is done automatically between the systems of the two sectors (in case of en-route control) . There are some cases which require audio coordination between a sector and another ( such as between en-route/approach and towers ) . When the automatic system fails then the controllers switch back to coordinating via voice .
The controllers do not pass electronic strips between them, the systems(FDPS) use a protocol to exchange data between them. In Europe the protocol used is called OLDI

In the US, ATC centers will have letters of agreement (LOA's) with adjacent centers. These will describe the best procedures to use when passing flights off (such as what altitudes and speeds the aircraft should be so as to make the process seamless. If everything is done according to the LOA's they won't have to call each other. The system will take care of displaying the correct data. If something is unusual they will need to physically call the next controller to explain what's going on and coordinate the handoff.


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What is the En-Route Information Display system (ERIDS) used for?
ERIDS is an information display system used at Air Route Traffic Control Centers. It provides electronic access to things like NOTAMs and static documents that previously were only available in paper. It was Implemented around 2006.
I get that it provides a bunch of general information and NOTAMs and PIREPs and such, but who uses ERIDS to access that? I know controllers have their own display, and I was under the impression that NOTAMs and such could be accessed there. I would assume other personnel also have their own displays. Is ERIDS just like a back-up source of information?

Just visited a facility and learned what this is.
ERIDS provides digital access to static documents like standard procedures and things of that nature. ERIDS provides easier access to these documents than would otherwise be possible. ERIDS also provides a way (not the only way) to access NOTAMs or PIREPS or send PIREPS. I know controllers have other ways to access this information; I am not sure about other personnel. Having multiple ways to access or send this information is helpful to controllers as there may be multiple controllers in a sector and they may need to both access these services.
Essentially, ERIDS is just an informational system that is easy to access in an air route traffic control facility.


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Does lateral stability decrease during transonic flight? If so, why?
I haven't got much more to explain other than the title of the question itself. This question was written down in my notes that I took while studying Principles of Flight for my EASA commercial license.

Ideally it would not. With increasing Mach number the lift curve slope of the vertical tail goes up proportional to the Prandtl-Glauert factor $\frac{1}{\sqrt{1-Ma^2}}$ while the destabilising fuselage as a slender body is mainly unaffected by Mach effects. But that is only theory which assumes a perfectly rigid body.
In reality, elasticity decreases the effectivity of the vertical tail. Higher loads lead to higher deformations which in turn decrease the loads and with them the stabilising moments. What happens in detail depends on the structure: A light aircraft will experience decreasing directional stability as dynamic pressure increases.
At supersonic speed, fin effectivity is further reduced by the now again decreasing lift curve slope as Mach increases further. Now even the rigid aircraft experiences decreasing directional stability and elasticity will reduce stability further.

Directional stability over Mach, from Ray Whitford's Fundamentals of Fighter Design

How is ve defined?

Wow, looks like normograh writing, I hadn't seen that since I left school :-)

@mns: I have only those figures, and they have no units. It is a qualitative measure of directional stability, that's all I know. British nomenclature is odd at times.

It depends- there are two things in play here and the net effect would be dependent  on their interaction.
The most important contributor to lateral stability is dihedral effect, $C_{l_{\beta}}$, which is not usually affected by high Mach numbers. However, in case of a highly effective vertical tail (which would be improving the lateral stability by introducing a rolling moment), the increase in lift curve slope of the vertical tail increases in its contribution, which would improve lateral stability.
However, lateral stability would degrade if the lift distribution over the wing is affected by shock wave formation. The net effect would depend on the magnitude of these things.

Change in lateral stability derivatives with Mach number, from Chapter on lateral-directional stability from USAF test pilots school.
It can be seen that the $C_{l_{\beta}}$ decreases in transonic regime, which improves lateral stability. However, at these speeds, aeroelastic considerations become important, leading to effects like aileron reversal and reduction in lateral control effectiveness due to flow separation as a result of shock formation.


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Why is the first turbine in a turboprop (or turboshaft) connected to the compressor?
Typically the first turbine after the combustion chamber drives the compressor as shown in this picture.

(wikimedia.org)
This first turbine spins the fastest and harvests the most energy from the gas flow. This seems inefficient when you want to maximize power output from the power turbine. So why not have the power turbine the first to take advantage of the gas flow and then the compressor/gas producer turbine second?

You can only use for shaft horse power what is available after the energy to drive the compressor is subtracted. This subtraction is done by the high pressure turbine. Any energy left in the exit flow after that can be extracted to drive a propeller, a fan or a generator.
What happens in your arrangement if the power turbine extracts too much energy to keep the core engine running? No, clearly the priority has to go to keeping the core engine alive and to only use what is left.

The compressor obviously requires a power source, in order to do the work of compressing the air before it is mixed with the fuel. The first turbine takes some of the energy from the combustion process and uses it to drive the compressor.


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Why isn't dumped fuel burned?
Fuel dumping seems to have a lot of restrictions, like the need to do it above a certain altitude and over water. It also has to end up somewhere, even if it vaporizes, which is less than desirable. It seems like burning off fuel as it's dumped could solve these problems. Obviously there's the risk of rapid unplanned incineration, but what are the other factors?

@Loong Seems it's not as hazardous as I thought

Why is fully vaporized fuel less desirable than burned fuel?

@Terry Aren't fuels somewhat toxic? Also, fuel takes time to vaporize, limiting the altitude it can be dumped at to fully vaporize before hitting the ground

Well, if it's vaporized, it doesn't hit the ground. And if it's vaporized, I'd think UV would oxidize it fairly rapidly.

Quoting from my comment on a different answer: "You dump several tons of petroleum into 5.5 quadrillion tons of air. Noone's gonna notice."

I'm pretty sure passengers would panic when they saw a trail of fire coming out of the aircraft. Remember that fuel dumping isn't a "normal day" operation, fuel dumping is done in an emergency, and is quite rare (aside from the USAF, which, in comparison, does it more often).

Mostly the rapid unplanned incineration part. That's pretty bad.

If a chemist is on board, it would be interesting to know if after a few days of interaction and UVs, the result is not the same: C02 and "DHMO".

There's no benefit to burning dumped fuel, it actually introduces risk to the airplane which is dumping fuel and any other aircraft in the vicinity and possibly people on the ground depending on the altitude at which is done. Building a system to burn it is not worth the time, effort and money given there's no good reason to do it.
Dumped fuel will vaporize and mix in with the air, and then break down over time from exposure to air, heat and UV light. This is not ideal for sure, but burning it isn't environmentally friendly either as it produces CO2, CO and soot. Evaporation of dumped fuel is a miniscule problem when looked at in context as it's such a rare occurrence. Canada measured 58 million liters of gasoline and diesel lost to evaporation from retail outlets and pump in 2009, if you extrapolate that to the US it's got to be over 100 million US gallons of fuel evaporating into the atmosphere every year. An airbus A380 contains 320,000 liters of fuel, if you dumped all that at once it's not going to make a statistical difference.

Something tells me that it would be awfully difficult to get the burning during a fuel dump anywhere near ideal, anyway. A lot of effort is spent on making engines as fuel-efficient as possible, which means ideal burning of the fuel. By basically dumping the fuel into the surrounding air and igniting it, you essentially have no control over the burning process at all. Add to this that the fuel will be dispersing quickly when dumped even at a relatively low few hundred km/h airspeed. It follows that I'd expect the burning to be highly inefficient at best.

It would be a technical challenge for sure, to keep the fuel burning constantly and protect the airplane.

I'd expect the eventual breakdown products probably include CO2, so from a greenhouse gas perspective most of the damage is probably done either way.

@aCVn I'd expect the fuel to burn inefficiently (in fact at 0% efficiency because it's, not producing any power or thrust), but I wouldn't make any assumptions about the actual burning itself.

Obviously there's the risk of rapid unplanned incineration, but what are the other factors?

"Rapid unplanned incineration", although appearing nowhere in the certification standards for any airplane, will be fairly high up on the list of thing aircraft designers try to avoid. The linked F-111 dump-and-burn is very much an exception that is highly unlikely to occur in any civilian aircraft.
Dumping fuel from higher altitudes has not yet produced a problem for anyone, burning it as it leaves will require extensive (and expensive) testing that no one is interested in. Save it for the air shows.

"fairly high up on the list of thing aircraft designers try to avoid" - indeed.

"The aircraft experienced an episode of rapid unplanned incineration" is like "The system experienced an uncontrolled thermal runaway" -- No matter how the engineer tries to dress it up it still means "THE DAMN THING CAUGHT FIRE!" and that's generally considered to be an undesirable characteristic. :)

@voretaq7 I suspect it would be an especially undesirable characteristic of a system moving at a few hundred km/h at some nontrivial distance above ground, where the system is a big metal tube packed with a few hundred people.

As GdD explained, while aircraft are supposed to be careful about where they dump fuel, the dumped fuel is not a significant hazard to health or the environment.
Burning the fuel represents much more risk than just "rapid unplanned incineration". Large airliners departing for a long flight can be carrying enough fuel to fly for 6 to 10 hours, and must dump this fuel in less than a few hours. All of the energy that would have gone into propelling the aircraft over a long period of time is released as heat in a much shorter amount of time. This heat will be a problem.
Fuel dump nozzles are typically placed on the wings. This allows the fuel to be dumped clear of the fuselage and engines, and is conveniently located right next to the fuel tanks in the wings. If the fuel is burned directly from these nozzles, it will cause some issues. The nozzles will have to be designed to withstand the heat. The wings in this area also need to be designed for this heat, and prevent the fuel (vapors) inside from igniting. The passengers would probably not be thrilled to see giant flames coming from the wings, and would definitely feel the heat.
The fuel dump nozzles could be moved but this adds complexity and risk. The other logical place would be near the APU in the tail of the aircraft. The tail would still be affected by the heat, and in some large aircraft there are still fuel tanks in the tail that must be dealt with.
All of these challenges, on top of controlling a massive fire outside of the aircraft, and burning the fuel provides relatively little benefit. Simply dumping unused fuel is much safer for everyone involved.


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What is the typical procedure if an engine fails during a vertical takeoff?
My understanding of VTOL aircraft (correct me if I am mistaken) is that the plane accelerates vertically until it's a bit off the ground, at which point the thrust is slowly vectored horizontally, and when it's finished, the plane has enough speed to produce sufficient lift. So, what happens if an engine fails before the plane has enough speed to lift itself? I'm asking about both single and double engine planes, for example the F-35, Harrier Jump Jet, or Yak 36.

I would suggest ejection. Fast.

What plane? This would be different for an F-35 or V-22.

@RonBeyer I was thinking along the lines of a jet like the F-35. I'll add that to the question

I wonder how much thought has been put into it when the airplane is designed. Ones like the Yak has independent engines front and aft and if one fails then the plane would certain just flip over instantly. Being military and especially Russian military pilots's lives is a resource, or even a consumable, so I imagine at the peak of cold war if they could save a reasonable chunk of time and money by increasing the pilot's death rate a littler they would probably do it. Just like what to do if the heat shield of space shuttle is damaged before reentry, the answer could be just "good luck"

Nothing good

Not sure about planes, but helicopters are also VTOL and, to borrow a line from the AW139 RFM
The H-V diagram defines, in the event of a single engine failure during take-off, landing or other operation near the surface, a combination of airspeed and height above ground from which a safe single engine landing on a smooth, level and hard surface cannot be assured

Although not during takeoff, there's a video here of the correct procedure when a hover is messed up - https://www.youtube.com/watch?v=ax5w_4r7kCI  --- note, "An RAF board of inquiry has now established that Flight Lieutenant Cann had accidentally operated the controls for throttle and nozzle direction lever at the same time causing it to drop like a stone."

@user3528438: The Yak-38's forward-mounted lift engines are mounted inline, so a failure of one wouldn't cause the aircraft to flip over (as would happen if they were mounted side-by-side).

F35 and Harrier are both single engined. Both  had a forward fan providing lift through this fan, and in case of Harrier forward nozzles turn aft for thrust. F35 it provided lift only.  Aft nozzles in both types vector to give lift or thrust. In Harrier all nozzles are synchronised,
So silence from the engine means it's time for the black and yellow handle.. If a twin engine e VTOL aircraft where lift is provided by both engines and one has a failure, then again until you transition to sufficient aerodynamic lift you are going down... But also probably rolling away due to non symmetric thust. Look at design of DornierDo31. You can climb I side on at the Deutches  Museum at Oberschleissheim near Munich.( I was Senior Systems Engineer on Harrier, Sea Harrier. )

The Yak-38 even had an automatic ejection if in the take-off mode it rolled over 60°—whether it was due to failure of one of the engines, sudden wind gust or incorrect handling, it was not recoverable either way and the pilot had to eject before the plane rolled over completely.

Noted, makes sense, Harrie had nothing like that, F35. Harrie r did have pedal shakers to cue pilot to sideslip (press the rudder pedal to reduce)   since a phenomenon called" intake momentum drag" at low speeds could cause yaw then roll divergence. It also has a yaw vane just ahead of the windshield to show pilot any sideslip. No associated  sensor, just a "weathervane"


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How often are drop tanks actually dropped?
I know drop tanks are used to extend the range on many fighter jets, but how often are they actually detached from the plane once empty? A lot of WWII movies and documentaries show tanks being dropped before entering combat, but I'm curious about more modern planes and non-combat conditions. Does the increased air resistance offset the cost of dropping the tank on say, a standard patrol, or are they landed and recovered with the plane and if so, are they re-used or simply scrapped?

I can't see any reason to drop the tanks unless it's a combat or emergency situation.

You should google 'drop tank boats' if you want to know what happens to discarded tanks.

Today's external tanks are not exactly cheap, so they are nearly always returned to base with the aircraft.
These tanks have "only" 2 drawbacks :

They add some weight (slightly increasing fuel consumption and limiting aircraft maneuverability, or how much Gs he can take)
They add some drag (slightly increasing fuel consumption and slightly decreasing performances like speed recovery, in case of a dogfight)

There are only 2 cases where the aircraft will jettison his external fuel tanks :

They are engaged by hostile SAMs or aircrafts and have to perform evasive maneuvers / air combat maneuvers, they will get rid of anything they won't need to survive and that affects aircraft performances (drop tanks, air-ground armament ...) to optimize their chances to survive.
Technical issue that would require clean wing / lighter aircraft (loss of thrust, landing gear issue, fuel leak ...)

Some aircrafts like french interceptor Mirage 2000-C have optimized fuel tanks (center line tank in this case), to affect as little as possible aircraft performances and allow max G once empty so they don't have to jettison it.

In general, unless it's an emergency or needed for certain flight operations, the tanks are retained and flown back to base.
There are maneuvering limitations for carriage of certain stores on some aircraft.  The F-16C/D for example limited maneuvering to +6.5/-3 Gs with stores on the inboard two pylons and/or centerline drop tank installed.  If the aircraft was attacked by an offensive flight of enemy fighters, one of the first items prior to commencing ACM with the threat was to jettison stores on these stations.  This allowed the aircraft to maneuver up to a maximum of +9 Gs as the fight commences.


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Do aircraft use some kind of stability control to keep the aircraft on the runway during landing?
Modern automobiles use an electronic system called ESP to keep the car on track in slippery situations. Do modern jet liners have a similar system to aid the pilot in keeping the aircraft on the runway after touchdown?
Especially during crosswind landings you can feel the plane swilling violently left/right after touchdown.

Autopilot landing is helpful as the rudder will be controlled by the autopilot to keep the aircraft on the runway.


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Why are parking brakes not used for parking jetliners?
From the question 'Do I Set the Parking Brake On the Ramp?' it is concluded that you do not set the parking brake when parking the aircraft.
The question concerns small aircraft which can be parked for a longer period of time. However, commercial jets (jetliners) are also parked at the gate without the parking brake engaged and rely on blocks/chocks placed around the wheels.
Then why is it called the "parking brake" as it is not used while parking the aircraft? Other wheeled vehicles like cars, trucks and buses are always parked on the parking brake.

"Other wheeled vehicles like cars, trucks and busses are always parked on the parking brake." Not always. With manual transmissions, some people prefer to leave their car in low gear (first or reverse, depending on whether they're parking uphill or downhill) rather than engaging the parking brake. With automatic transmission, "park" is separate from the parking brake and can be used independently. My understanding is that this is particularly common in cold climates to keep the parking brake from freezing. I'm pretty sure I've seen mention of this on [mechanics.se], but can't find it now.

My guess is, you can not really engage the hot bake while standing still for too long. The friction material would bind to the brake surface. I've heard of advice that when a motorcycle comes to a stop light, the rider should use the rear brake while waiting for the light and release the front brake, or the brake pads would bind to the hot disk and leave some residue that can cause the front brake to "pulse". Aircraft brakes may suffer from the same problem, so the parking brake could be only use when the brake is cold.

@MichaelKjörling is correct. Automatic transmission vehicles normally do not set the parking brake when parking. Instead, the transmission holds the vehicle in place when 'park' is selected.

@reirab: And then you set the parking brake manually, just to make sure.

@Sean I almost never do that in an automatic unless I'm parked on a steep grade. There's no point. The transmission is plenty to hold the vehicle in place unless the grade is quite steep. Of course, I do set the parking brake when parking a light airplane, as they have no transmission and the wheels will otherwise roll quite freely.

Parking brakes are used by commercial aircraft at the gate if the ground crew can not set the chocks. The most common time for this to happen is during a lightning storm when ground crews are not allowed on the ramp. Large airports like DFW have automatic parking lights that will guide the aircraft to the proper alignment and stopping point. The crew will then set the parking brake before deplaning the passengers. Some crews will also use the parking brake for ramp holds, de-icing, etc.
The reason why chocks are preferred over the parking brake, is due to the way parking brakes work on aircraft. On commercial aircraft, the pilots apply the brake manually, then turn on the parking brake, which closes a hydraulic valve trapping the pressurized fluid in the brakes. Due to internal leakage of the valve, the brake pressure will slowly bleed down over a few hours. Therefore, after a few hours the brakes will no longer hold the aircraft. Further, most aircraft ramps are slightly sloped away from the terminal, so the aircraft would end up rolling backwards. Due to the bleed down issues, most airlines require pilots to stay in the cockpit ready to apply brakes until the wheel are chocked.
I have personally seen cases of large aircraft being parked with a parking brake, that rolled after the brake bleed down. In one case, a 777 rolled backwards and the jet-bridge tore off its door. Link to News Article
Car parking brakes work differently. Parking brakes in cars have a direct mechanical linkage (typically a cable) that is held in place. Therefore, they do not have the bleed down issue and will maintain the same amount of braking force at all times.

Ditto for "direct mechanical leakage". I think you meant "linkage".

Does DFW allow the jet bridge operators to work during lightning storms? When we had a lightning storm start at MCO just after we had boarded, but before pushing back, we all just had to sit there for a couple of hours because the ramp closure meant that the tugs couldn't push us back and also that the jet bridge operator couldn't re-attach the bridge until after the lightning stopped.

I am going to answer you from an airline pilot point of view. First of all, parking brakes in aircraft work pretty similar to the ones in a car or a truck. And yes they are used to park the airplane by letting go off the toe brakes, which is again pretty similar to a road vehicle.
Once we are at the gate after a flight, we put on the parking brake and shut the aircraft down. The ground engineer usually lets us know through the intercom once the chocks are in place. This is a sort of 'okay' signal to release the parking brake. The reason why we use chocks and keep the parking brake off is mainly to cool down the brakes. Following a typical landing, and sometimes a long taxi to the gate, the brakes tend to heat up. Because airplanes rarely stay on the ground, we need to cool the brakes down to ensure they remain efficient in a possible rejected take off that might occur in the next flight. Keeping the parking brake off makes sure that the brake stators are released from the rotors, allowing better air flow circulation. This ensures proper heat dissipation. In some airplanes, we have brake fans, which helps to cool the brakes. The fan is more effective with the parking brake off. However, for the walkaround, we set the park brake on, even if the chocks are in place. This is so that we can get a more accurate indication of the brake wear indicators.
When the airplane has to be parked at a place for a long period of time, the parking brake can be kept on. But as the parking brake in most airplanes (when unpowered) is channeled through a pressurized hydraulic accumulator, it tends to slowly lose pressure. For example, the A320 accumulator can hold the brakes in place for about 12 hours. If you have to park it for more than 12 hours, you should place the chocks on the wheels to prevent it from moving and hitting something.

The A320 parking brake system. It is powered by the yellow hydraulic system and by an accumulator when the aircraft is off power.

It’s so a linesman can tow your plane around if need be without you being there or needing to get into the plane.

I understand that this applies to smaller aircraft but the larger commercial aircraft stay at the same gate in general.

@Brilsmurfffje Perhaps that may be true in the majority of cases, but there are significant exceptions. For example, aircraft coming into the terminal for overseas arrivals at JFK are always towed to their respective company terminals after discharging their pax. And the last 747 carrier I flew for had only 3 gates at their headquarters terminal and almost always towed from the gate. Also nose-loading 747 freighters are typically towed away from nose loading ports that lead into a cargo facility.

I would add that chocks are a more reliable - and visible - way to ensure a plane is actually held in place, whether GA or much larger.

@Terry using normal tugs, almost all airlines require a brake rider in the cockpit during towing (to apply breaks if the tow bar disconnects), who could release the parking brake. High speed tugs don't always have to have a brake rider, though.

@JohnPolson Be aware that if you have posted a question, it's not at all frowned upon to edit your answer to make it more complete rather than putting it in a comment.

@Brilsmurfffje aircraft are regularly towed from gates to maintenance areas overnight or if their return flight is long enough away that keeping them at the gate would obstruct other flights.